No argument is intended, just trying to share what I have learned here.
I have not constructed a PMA with non-ferrous rotors. I trust the poor results of the "Wood AX" a wooden rotor machine (I believe that was the name) that the owners of this board built in the distant past. I also trust such experts such as Hugh Piggott, Flux and the owners of this board who have built countless PMA's and provided extensive results from their testing. All indications are that your magnets are probably shorting the flux between adjacent magnets on the same rotor instead of the flux crossing the gap that has copper in it. You don't need bigger magnets or more efficient coil geometries. You need to "channel" the flux you have across the gap between rotors. If there is a way to do this without a ferrous metal backing I do not know how it could be done. The mentioned experts also will tell you that you can build multiple stator machines but they do not use materials as efficiently as a single stator dual steel rotor machine i.e. using the same materials your machine will have a greater output in the single stator, dual rotor design.
I absolutely applaud your efforts. You are a builder not a armchair engineer as shown by your real life builds, however, it is not necessarily a wise thing to ignore other peoples mistakes and repeat them.
Yes what Ghurd said Outer rotors in a multiple stator machine should be a ferrous material.
Thurmond